Whole-wheat sweet potato muffins

Ingredients

• 1/2 pound (about 2 small) sweet potatoes
• Vegetable oil spray for coating the tins
• 1/4 cup ( 1/2 stick) butter
• 3 tablespoons dark brown sugar
• 3 tablespoons sugar
• 1 cup flour
• 3/4 cup whole-wheat flour
• 1/4 cup whole-wheat pastry flour
• 1 teaspoon baking powder
• 1/2 teaspoon baking soda
• 1/2 teaspoon salt
• 1 tablespoon cinnamon
• 1 teaspoon ginger
• 1/2 teaspoon allspice
• 1 cup buttermilk
• 1/4 cup plain yogurt
• 1 egg
• 1 teaspoon vanilla
• 12 Medjool dates, pitted and cut into 1/4 - to 1/2 -inch pieces